This invention relates to frequency division multiplex (FDM) systems and, more particularly, to improved method and apparatus for generating modulated line signals.
Frequency division multiplexing is the process of simultaneously transmitting a plurality of voice telephone conversations over a pair of wires, each of the voice signals being shifted to a higher preassigned unique position in the frequency spectrum for subsequent transmission. In one prior art FDM system, the input signal in each voice channel is combined in an associated first modulator with the same high frequency (HF) carrier signal having a frequency f.sub.c to produce a modulated signal comprising upper and lower sidebands of f.sub.c. The upper sideband is selected in each channel by an associated polylithic crystal bandpass filter, all of which have the same passband. The carrier signal is also applied to a gate which is pulsed at a rate f.sub.p = 4 kHz to produce components having frequencies af.sub.c .+-. bf.sub.p, where a and b are integers. In each channel, a different one of these frequency components is selected by a notch filter and combined in a second modulator with the selected sideband to translate the latter to a prescribed-unique position in the frequency spectrum for subsequent transmission. It is relatively expensive to provide a high frequency notch filter having a different pick-off frequency in each channel. Another prior art FDM system, which is similar to the above, uses separate oscillators operating at slightly different frequencies in each channel in place of the notch filter, gate, and pulse signal source. In order to prevent drift and subsequent impairment of recovered voice signals, rigid stability requirements are placed on the carrier oscillators which makes them relatively expensive. In yet another prior art FDM system, an HF carrier signal of frequency f.sub.c is pulsed at a low frequency rate f.sub.p to produce carrier signal components having frequencies af.sub.c .+-. bf.sub.p. A notch filter in each channel picks off a different carrier component which is combined in an associated first modulator with the voice signal to produce sidebands of only that carrier component frequency. Polylithic crystal filters having different passbands select the upper sideband in each channel. A different carrier component is picked off by another notch filter and combined with each selected sideband in associated second modulators to translate the latter to prescribed unique positions in the frequency spectrum for subsequent transmission. Notch filters having different pick-off frequencies here add additional expense to this system.
The U.S. Pat. No. 3,518,376, issued June 30, 1970, entitled Low Frequency Television System by Ira Kamen et al, describes a communication system in which a different low frequency carrier signal is combined in each channel with the input signal there to produce sidebands of only the associated carrier frequency, one of the sidebands being selected at low frequencies by a bandpass filter for subsequent transmission. Low frequency bandpass filters are generally more expensive than high frequency polylithic crystal bandpass filters having the same bandwidths. The article, A Method For Frequency-Division Multiplexing and Its Integration With Time-Division Switching by P. M. Thrasher, IBM Journal, March 1965, pgs. 137-140, discloses a resonant transfer multiplex system. In this system, connecting each input voice signal to an associated bandpass filter at a prescribed low frequency sampling rate of f.sub.p = 8 kHz produces at the input to the filter sidebands of frequencies which are multiples of f.sub.p. Each bandpass filter selects a different-particular sideband. Since the switching occurs at low frequency, the filtering must also be accomplished at low frequencies. Bandpass filters having low frequency passbands are relatively expensive and complex when compared to high frequency polylithic crystal bandpass filters. Another resonant transfer multiplex system, which accomplishes bandpass filtering in each channel at low frequencies, is described in U.S. Pat. No. 3,859,469, issued Jan. 7, 1975, entitled Combination Hybrid Frequency Division Multiplexing Circuit by L. E. Getgen.
An object of this invention is the provision of improved FDM method and apparatus for producing sidebands of a plurality of harmonically related frequencies in each channel for selecting a different sideband in each channel for subsequent transmission.